Also, a sequel sort of implies that the plotline of the first story wraps up neatly, which is not always the case with the first book of a duology. Besides, it's easier to say 'duology' than it is to say 'book and sequel.' If you want something to pule and micturate about with 'duology' you could always use the fact that it's mixed Latin and Greek. Some folk seem to think that's a punishable offense.